Once upon a time, operating systems used to matter a lot; they defined what a computer could and couldn't do... Today, there's only one operating system: Unix (okay, there are two, but we'll get to that). This is why I contend that the OS doesn't matter - or that we need to take another look at the word's content, at what we mean when we say 'Operating System'.

-The security model (enforced at kernel level) does not matter either. The user/admin model is perfect for desktop and mobile space applications. If an application has access to all of the user's precious data or only has to ask for an extremely vague "privileged access" to get access it, it's okay as long as the UAC window is nicely designed.

A warning is due; be prepared when discussing application capabilities (rights) in the churchs (some channels) of IRC servers, as any idea deviating from the traditional model of applications (processes) having unrestricted access to the home directory of associated user, is the sign of the anti-christ.
Galileo, watch out !